Moving coil measuring instrument



y 3, 1950 1.. J. LUNAS 2,508,410

MOVING COIL MEASURING INSTRUMENT Filed Dec. 27, 1944 4 Sheets-Sheet l INVENTOR -ZM/:%Yra/Z(/He5,

ATTORNEY May 23, 1950 L. J. LUNAS MOVING COIL MEASURING INSTRUMENT 4 Sheets-Sheet 2 Filed Dec. 27, 1944 INVENTOR [kw/7:6 [u /2:25.

W TNESSES:

BY Kim ATTORNEY May 23, 1950 L. J. LUNAS MOVING COIL MEASURING INSTRUMENT 4 Sheets-Sheet 3 Filed Dec. 27, 1944 INVENTOR Zfil//r?(?. 0172 Z'fdW ATTORNEY JZ/ZZ WITNESSES:

y 3, 1950 L. J. LUNAS 2,508,410

MOVING COIL MEASURING INSTRUMENT Filed Dec. 27, 1944 4 Sheets-Sheet 4 A a-0461f i0 java/91:6.

WITNESSESI INVENTOR fd/wamjf 0x 45,

KKMW ATTORNEY @M/ f j l atented May 23,

UNITED STATES PATENT OFFICE MOVING COIL MEASURING INSTRUMENT Application December 27, 1944, Serial No. 570,028

47 Claims. 1

This invention relates to electrical instruments and it has particular relation to electrical measuring instruments of the electrodynamic type.

As well understood in the art, an electrodynamic instrument includes one or more movable coils rotatably mounted with respect to one or more fixed windings. Depending on the type of energization employed, the electrodynamic instrument may be responsive to various variable electric-a1 quantities such as voltage, current and power. The electrodynamic instrument may be energized either with direct current or with alternating current, and may be employed as a relay for actuating relay contacts, or as a measuring instrument either of the indicating type or of the recording type. The invention is directed primarily to an electrodynamic instrument having a movable coil mounted for rotation about an axis intermediate two sides of the coil, both of the sides being employed in developing torque for actuating the instrument.

In the prior art, an electrodynamic instrument of the foregoing type has been provided with a magnetic structure to form what may be termed an ironclad instrument. This magnetic structure provided a path for magnetic flux, but it included an air gap of such great length that the magnetic structure added little to the efficiency of the instrument. As a matter of fact, the magnetic structure was more effective as a shield than as a device for increasing the efiiciency of the instrument. Furthermore, the magnetic structure employed in the prior art was formed in two or more parts which must be assembled or disassembled during the assembling or disassembling of the instrument. The multipart construction is objectionable for the reasons that it complicates the construction and maintenance of the instrument and it renders diflicult the provision of accurate air gaps.

In accordance with the invention, an electrodynamic instrument of the above-mentioned type is provided with a magnetic structure permanently assembled and preferably of one-piece construction. The magnetic structure has a magnetic core passing through the movable coil, but this core is provided with a passage through which the movable coil may be inserted in or removed from operative position with respect thereto. In addition, a small air gap is employed which results in an extremely efficient instrument.

In order to provide certain desired characteristics in the instrument, a magnetic part is employed which is asymmetric with respect to the path of travel of the movable coil. Consequently, when the movable coil alone is energized, the movable coil tends to seek a position wherein the magnetic reluctance of the magnetic path provided for magnetic flux produced by current flowing in the movable coil is a minimum. This may be termed a solenoid action, and introduces a source of error which cannot be readily corrected in certain cases by calibration.

In accordance with a further aspect of the invention, the solenoid action is substantially eliminated by providing a magnetic structure for an electrodynamic instrument which is formed of two magnetic parts or sections, each asymmetric with respect to the path of travel of the movable coil. The magnetic sections are so constructed and located that the resultant magnetic structure formed thereby is substantially symmetric with respect to the path of travel of the movable coil. For this reason the solenoid action introduced by one of the magnetic sections is substantially compensated by the solenoid action introduced by the other of the magnetic sections.

The magnetic sections have passages extending therethrough to facilitate removal of the movable coil from the magnetic structure. However, since the magnetic sections have their asymmetries oppositely disposed with respect to the path of the movable coil, the passages in the sections are not in alignment. To permit withdrawal and insertion of the movable coil with respect to the magnetic structure, the magnetic sections are spaced apart in the direction of the axis of rotation of the movable coil by a distance suflicient to permit movement of a side of the coil therebetween. By suitable manipulation the movable coil may be passed through the resultant passage formed by the passages in the sections and the space between the sections.

The invention also contemplates the provision of two elements, each including a magnetic structure and a movable coil with the movable coils of the two elements mounted inv alignment on a common shaft. The elements are spaced apart having a movable coil mounted for rotation with respect to a fixed winding about an axis which is intermediate two sides of the movable coil.

It is a further object of the invention to provide an electrodynamic instrument having a unitary magnetic structure providing a separate air gap for each of two sides of a movable coil.

It is another object of the invention to provide an electrodynamic instrument having a magnetic structure formed of two magnetic sections each asymmetric with respect to the path of travel of the movable coil of the instrument and having their asymmetries so disposed that the resultant magnetic structure is substantially symmetric with respect to the path of travel of the movable coil.

It is another object of the invention to provide an electrodynamic instrument having a magnetic structure through which a movable coil may be inserted into operative position or removed therefrom without disturbing the magnetic structure.

It is a further object of the invention to provide an electrodynamic instrument having a magnetic structure through which a movable coil may be inserted in mounted position or removed therefrom without disturbance to the magnetic structure, the magnetic structure having substantially no solenoid action with respect to the movable coil.

It is a still further object of the invention to provide a two-element electrodynamic instrument having magnetic structures in which a pair of movable coils may be inserted in or removed from mounted position without disturbance to the magnetic structures and wherein the magnetic structures have substantially no solenoid action with respect to the movable coils.

It is an additional object ef the invention to provide an improved electrodynamic instrument which is substantially immune to externallyproduced magnetic field influence.

Other objects will be apparent from the following description taken in conjunction with the accompanying drawings, in which:

Figure 1 is a schematic view of an electrodynamic instrument having a magnetic core;

Figs. 2 to 9, inclusive, are schematic views of embodiments of electrodynamic instruments which illustrate the invention;

Figs. to 15, inclusive, are schematic views showing connections of moving coil assemblies suitable for certain ofthe electrodynamic instruments illustrated in Figs. 2 to 9, inclusive;

Fig. 16 is a view in perspective with parts exploded of an electrodynamic instrument embodying the invention;

Fig. 17 is a view in perspective with parts broken away of the instrument shown in Fig. 16;

Fig. 18 is a view in side elevation of a twoelement electrodynamic instrument embodying the invention;

Fig. 19 is a schematic view of circuit connections suitable for the instrument of Fig. 16;

Fig. 20 is a view in sectional elevation with parts broken away showing winding insulation suitable for the instrument of Fig. 16;

Fig. 21 is a detailed view in perspective of a portion of the insulation illustrated in Fig. 20;

Fig. 22 is a View in sectional elevation with parts broken away of a two-element electrodynamic instrument embodying the invention;

Fig. 23 is a view in perspective of a damping magnet assembly suitable for the instrument of F 2;

Fig. 2 4 is a detailed view in perspective of a about the axis of the shaft.

bracket employed in the instrument of Fig. 22; and

Figs. 25 to 28, inclusive, are views in perspective of further embodiments of electrodynamic instruments embodying the invention.

Referring to the drawings, Fig. 1 shows an electrodynamic electrical instrument which includes a cylindrical magnetic core 2 and a pair of outer magnetic pole pieces 4 and 6 which are spaced from the magnetic core 2 to define therewith a pair of air gaps 8 and I8. The magnetic core and the pole pieces form parts of a magnetic structure which is completed by a magnetic conductor l2. The magnetic core, the pole pieces and the magnetic conductor may be formed of a soft magnetic material, such as silicon iron. A winding [4 surrounds the conductor l2 and, when energized by direct current produces a flow of magnetic flux in the direction indicated by the arrow 4:. With the direction of flow of magnetic flux illustrated in Fig. 1, the pole piece 6 may be termed a north pole indicated by the reference character N, whereas the pole piece 4 is a south pole S. If the winding [4 is energized by alternating current, the arrow 1) and reference characters N and S indicate instantaneous conditions.

A moving coil l6 surrounds the cylindrical magnetic core 2 and is secured to a shaft [8 for rotation about the axis of the shaft. This moving coil has one side 20 positioned for movement through the air gap 8 and a second side 22 positioned for movement through the air gap II]. It will be observed that the shaft I8 is positioned intermediate the two sides 20 and 22. As well understood in the art, when an electrical current is directed through the coil Hi, the current flowing through the coil side 20 coacts with the magnetic field in the air gap 8 to produce a torque acting on the coil IS about the shaft Hi. In addition, current flowing through the coil side 22 produces an additional'torque acting on the coil about the shaft l8. Movement of the coil in response to the resultant torque acting thereon is opposed by a spring (not shown in Fig. 1). As also understood in the art the winding 14 and coil I6 may both be energized by direct current or both may be energized by alternating current.

As previously pointed out, the removal of the moving coil it from the instrument. illustrated in Fig. 1 requires the removal of the magnetic core '2 from the remainer ofthe magnetic structure. The removal of the magnetic core 2 is objectionabl for the reasons that it tends to change the resultant characteristics of the magnetic structure upon reassembly thereof, it increases the possibility of damage to the moving coil and the pivots associated therewith and it necessitates the realignment of the magnetic core with the remainder of the instrument after each reassembly thereof.

To facilitate assembly and disassembly of the instrument, the magnetic core 2 is replaced by two magnetic parts 24 and 26 (Fig. 2 which are spaced apart to provide. a passage through which a moving coil may be moved. The magnetic parts 24 and 26 may be termed magnetic cores which coast to form a resultant magnetic core or they maybe termed inner magnetic pole pieces. The magnetic structure of Fig. 2 otherwise is similar to that of Fig. 1.

In Fig. 2, a coil assembly 28 is provided which corresponds to the moving coil i Fig. 1. This moving coil is mounted on a shaft 30 for rotation If desired, the coil assembly 28 may be similar to the moving coil I6 of Fig. 1. However, since the inner pole pieces 24 and 26 are separated from each other, the coil assembly 28 may be formed of two moving coils each embracing a separate One of the inner pole pieces 24 and 26. Such a coil assembly will be illustrated and discussed below. It should be noted that the shaft 3|) may be a through shaft extending completely through the space between the inner pole pieces 24 and 26.

The space between the inner pole pieces 24 and 26 provides a passage 32 which is proportioned to permit movement of the coil assembly 28 therethrough from a position external to the magnetic structure to a position wherein the coil may be rotated to advance its sides 34 and 36 into the associated air gaps 8 and II). This greatly facilitates the assembly and disassembly of the instrument. For example, let it be assumed that the coil assembly 28 is to be removed from its associated magnetic structure. To effect such removal, the coil assembly 2 8 may be rotated in a counterclockwise direction, as viewed in Fig. 2, from the position illustrated in full lines through a predetermined path to the position illustrated in broken lines. When the coil is in the position illustrated in broken lines, it may be moved through the passage 32 in a direction parallel to the shaft st or transverse thereto to a position external to the magnetic structure. It should be noted that such removal of the coil does not disturb the magnetic circuit in any way. Consequently, the objections noted with respect to Fig. 1 are completely avoided. An opposite procedure may be followed to reinsert the coil assembly 28 in its operative position with respect to its associated magnetic structure.

The separation of the inner pOle pieces 24 and 2s introduces a very appreciable air gap represented by the passage 32 into the magnetic circuit through which the magnetic flux 3 flows. Although the instrument of Fig. 2 is operative, the -resence of a large air gap in the magnetic circuit is not desirable. Such an air gap introduces a large magnetic reluctance in the magnetic circuit, and decreases the efliciency of the magnetic circuit.

To eliminate the air gap represented by the passage 32 from the magnetic circuit established by the magnetic structure illustrated in Fig. 2, the connections of the magnetic circuit may be modir'ied. The presence of two completely independent pole pieces 2t and 26 between the outer pole pieces a and 8 permits a substantial variety in the connections of the magnetic circuit. For example, the air gaps 8 and It may be connected in series, in parallel, or in independent magnetic circuits as desired. Suitable connections for the pole pieces are illustrated in Figs. 3 to 8.

Referring to Fig. 3, it will be observed that the outer pole pieces i and 6 again are connected by the magnetic conductor I2. In addition, the in-- ner pole pieces 38 and 40, which correspond to the inner pole pieces 24 and 26 of Fig. 2, are connected by means of a magnetic conductor 42. Since the inner pole pieces 38 and 40 are magnetically connected, the passage 44 therebetween may be made substantially larger than the passage 32 of Fig. 2 without appreciably increasing the magnetic reluctance of the resulting magnetic circuit. By inspection of Fig. 3, it will be observed that the air gaps 8 and ID are connected in series, the direction of flux in the associated magnetic circuit being illustrated by the arrows for direct-current operation (instantaneous direction for alternating-current operation). For example, magnetic flux flows from the inner pole piece 38, which may be termed a north pole N through the air gap 8, the outer pole piece 4, which may be termed a south pole S, the magnetic conductor l2, the outer pole piece 6, which may be termed a north pole N, the inner pole piece 40 which may be termed a south pole S and the conductor 42 to the inner pole piece 38. Energization of the winding It by direct current or alternating current, as the case may be, produces the flux represented by the arrow It will be observed that the directions of flux flow in the air gaps 8 and ID are similar to the directions of magnetic flux flow in the air gaps of Fig. 1. Therefore, the torques which are applied to the coil it of Fig. 3 when current flows therethrough are similar to the torques applied to the coil l6 of Fig. 1 when the latter is energized. Since the magnetic conductors l2 and 42 are so disposed that they are external to the moving coil 16 when the moving coil is in the position indicated by broken lines in Fig. 3 and since they are displaced from the path of the moving coil as it rotates in a counterclockwise direction from the position illustrated in full lines in Fig. 3 to the position illustrated in broken lines therein, the magnetic conductors do not interfere with the removal of the coil from the associated magnetic structure or the reinsertion of the magnetic coil in its magnetic structure. Such removal or reinsertion may be accomplished in the manner discussed with reference to Fig. 2.

Fig. 4 also discloses a circuit wherein the air gaps 3 and it are connected in series in a magnetic circuit, but the connections differ slightly from those illustrated in Fig. 3. In Fig. 4, the inner pole piece 38 is connected by a magnetic conductor 4t to the outer pole piece 6 whereas the inner pole piece 40 is connected by a magnetic conductor iii to the outer pole piece 4. With the directions of fiux flow indicated by the arrows magnetic flux flows from the inner pole piece through the air gap 8, the outer pole piece 4, the magnetic conductor 48, the inner pole piece 4c, the air gap It, the outer pole piece 6 and the magnetic conductor 46 to the inner pole piece 38. The winding :4 may be energized by direct current for producing a flow or magnetic flux in the directions indicated by the arrows c. For alternating-current operation th arrows indicate instantaneous directions of flow.

It will be observed that the direction of magnetic flux flow in the air gap 8 is similar to that in the air gap 8 of Fig. 1. However, the direction of flow of magnetic flux in the air gap ill of Fig. i is opposite to that of the magnetic flux in the air gap ll] of Fig. 1. Consequently, if the coil l6 of Fig. 1 wer employed with the magnetic structure of Fig. 4, the torques applied to the coil would be in opposite directions about the shaft 36. For this reason, the magnetic structure of Fig. 4 is provided with a pair of moving coils so and 52 each of which is associated with a separate one of the air gaps 8 and H]. For example, the moving coil 50 has a side 54 disposed for movement through the air gap 8 whereas the moving coil 52 has a side 56 disposed for movement through the air gap II]. By suitable connections of the coils, the torques applied thereto when the coils are energized may be cumulative or differential, as desired. For example, the coils may be connected in series to apply torques cumulatively to the shaft 30. On the other hand, the coils may be energized from separate sources of electrical energy to apply their torques either cumulatively or differentially to the shaft 30. These connections of the coils will be discussed at greater length below. Since the magnetic conductors 48 and #58 are positioned to permit movement of the moving coils t and '52 in a counterclockwise direction from the position illustrated in full lines to the position illustrated in broken lines in Fig. 4 and since they do not pass through either of the moving coils when the moving coils are in the positions indicated by broken lines, the moving coil assembly may be removed from its associated magnetic structure and reinserted therein in the manner discussed with reference to Fig. 2.

With reference to the modification illustrated in Fig. 5, the air gaps 8 and it are connected in parallel in a magnetic circuit. For example, the inner pole piece 38 and the outer pole piece 6 are connected by magnetic conductors 58 and 60' to one end of a magnetic conductor 52. The outer pole piece 4 and the inner pole piece 40 are connected by magnetic conductors 64 and 68 to the remaining end of the magnetic conductor 82. The directions of magnetic flux flow for directcurrent energization of the winding M (or instantaneous directions for alternating-current energization) are indicated by arrows g5. It will be observed that magnetic flux from the magnetic conductor 82 flows through the magnetic conductors 58 and 69 to the inner pole piece 38 and the outer pole piece '3 which may be termed north poles N. From these pole pieces the magneticflux travels across the air gaps 8 and It to theouter pole piece 4 and the inner pole piece 48.. Consequently, the air gaps 8 and W in effect are connected in parallel between the ends of the magnetic conductor t2. Since the directions of flow of magnetic flux in the air gaps i3 and I5! are similar to the directions of flow of magnetic flux in the air gaps s and it of Fig. 1, the coil H5 may be. employed in Fig. 5 and will operate in a manner similar to its operation in the instrument of Fig. l. The magnetic conductors 58, 6B, 54 and 86 are located to permit removal of the coil E6 in. the manner discussed with reference to Figs. 2,. 3 and 4. Any portion of the pole pieces and magnetic conductors of Fig. 5 may have a winding associated therewith, such as the winding M, which when energized is capable of directing magnetic flux in the directions illustrated by the arrows s.

In Fig. 6, the air gaps 8 and H] are connected in parallel between the ends of a magnetic conductor 68. For example, the inner pole piece 38 and the inner pole piece it are both connected to one end of the magnetic conductor 68 through magnetic conductors l8 and 12. The outer magnetic pole pieces 4 and 6 are connected through magnetic conductors M and 76 to the remaining end of the magnetic conductor 68. Suitable directions of flow of magnetic flux are indicated by the arrows 5. It will be observed that the directions of magnetic flux flow in the air gaps 8 and iii are similar to the corresponding directions for the instrument of Fig. 4. Consequently, the coils 5d and 52 may be employed for the instrument of Fig. 6. The magnetic conductors 68, H3, 72, T4 and T6 are so located that they permit removal of the coils 53 and 52 in the instrument of Fig. 6 in the same manner discussed with reference to Fig. 4. Any portion of the pole pieces or magnetic conductors may have associated therewith a winding such as the winding H on the conductor 68 which is capable of directing magnetic flux in the directions illustrated by the arrows In Fig. '7, the air gaps 8 and 10 are provided with independent magnetic circuits for independent magnetic energization. The pole pieces 4 and 38 are connected by a magnetic conductor 18, whereas the pole pieces 6 and 40 are connected by a magnetic conductor 8!]. Suitable directions of flow of magnetic flux are indicated by the arrows adjacent the magnetic conductors. For the specific directions of flow of magnetic flux indicated in Fig. 7, the directions of fiowof magnetic flux in the air gaps 8 and I0 are similar to the directions of flow of magnetic flux in the air gaps 8 and ll) of Fig. 1. Consequently, the coil l6 may be employed in Fig. 7 in the same manner discussed with reference to Fig. 1. In addition, since the inner pole pieces 38 and 49 are separated, the coil It may be removed or inserted with respect to its associated magnetic structure in the manner discussed with reference to Fig. 2. Any desired portion of the magnetic circuit which includes the pole pieces 4 and 38 and the magnetic conductor 18 may have associated therewith a winding such as the winding Ma on the conductor '58, which, when energized, is capable of directing magnetic flux in the direction indicated by the arrow 1). Similarly, any portion of the magnetic circuit which includes the pole pieces and 4B and the magnetic conductor {it may have associated therewith a winding, such as the winding 54?) on the magnetic conductor 3:13, which, when energized, is capable of directing flux in the indicated direction. As previously explained, for alternating-current energization of the windings the arrows indicate instantaneous directions.

By reversing the direction of flow of magnetic flux in the magnetic conductor 80, the direction of flow of magnetic flux in the air gap It also is reversed. Such reversal is illustrated in Fig. 8. The directions of flow of magnetic flux in the air gaps t and it of Fig. 8 are similar to those in the air gaps 8 and it of Fig. 4. Consequently, the coils 58 and 52 are employed in the embodiment of Fig. 8. These coils may be removed from their associated magnetic structures in the manner discussed with reference to Fig. 4.

The pole pi ce constructions illustrated in Figs. 2 to 8 lend themselves admirably to the construction of a ratio-type instrument. For example, in Fig. 9 an inner pole piece 38a is provided which corresponds to the inner pole piece 38 of Figs. 3 to 8. However, the inner pole piece 38a is configured to provide, in cooperation with the outer pole piece 4, an air gap 8a which tapers from a large length adjacent its upper end (as viewed in Fig. 9) to a length adjacent its lower end. In an analogous manner, an inner pole piece 2 6a is provided which cooperates with the pole piece 6 to provide an air gap which tapers from a large length adjacent its upper end to a smaller length adjacent its lower end. The coils 50 and 52 are associated with these tapered air gaps. When the pole pieces of Fig. 9 are energized in accordance with the teachings of any of the modifications of Figs. 3 to 8, and the coils 59 and 52 are energized from independent sources of energy to appiy torques differentially to the shaft 38, the coil assembly takes a position corresponding to the ratio between the energizations of the two coils.

To illustrate the operation of a ratio-type instrument, let it be assumed that the coil 56 is energized from one source to produce a torque acting in the direction of the arrow 82 and that the coil 52 is energized from another source to produce a torque acting in the direction of the arrow 84. Let it be assumed further that the coils 59 and 52 are similar in construction. It will be noted that the coil 50 has a side disposed in the gap 8a at a position wherein the gap has a short length and consequently a relatively strong magnetic field. On the other hand, the coil 52 has a side disposed in the gap Illa which is in a position wherein the gap has a substantial length and a relatively weak magnetic field. If the currents supplied to the coils 50 and 52 are equal, and the coils are in the position illustrated in full lines in Fig. 9, the torque developed by the coil 59 is substantially greater than that developed by the coil 52. Consequently, the coil assembly will rotate in a clockwise direction to move the coil 59 gradually into a weaker part of the magnetic field in the air gap 8a as the coil 52 moves into a stronger part of the magnetic field in the air gap lila. This rotation continues until the torques developed by the two coils are equal. This condition is assumed to exist when the coils are in the position illustrated in broken lines in Fig. 9. If the ratio of the current in the coil 55 to that in the coil 52 increases above unity, the coil assembly will rotate still further in a clockwise direction until the torques developed by the coils again are equal. Consequently, the instrument of Fig. 9 may be calibrated to indicate the ratio between these two currents.

As above pointed out, sometimes it is expedient to use a pair of moving coils 50 and 52 in place of the single moving coil l6. Connections suitable for the moving coils 59 and 52 are illustrated in Figs. 10 to 15. In Fig. 10, the two coils 50 and 52 are connected in series between two terminals 86 and 88 throughflexible conductor strips 90. It will be recalled that the coil 50 has a side disposed in the air gap 8, whereas the coil 52 has a side disposed in the air gap ID. The connection in Fig. 10 is assumed to be such that the currents flowing in these two sides flow in opposite directions. Such a connection of the two coils i! and 52 is suitable when the coils are employed with the magnetic structures illustrated in Figs. 2, 3, 5 and '7. The same directions of current flow through the coils may be obtained by connecting them in parallel between the terminals 86 and 88, as illustrated in Fig. 11. Consequently, the connections illustrated in Fig. 11 also are suitable for the magnetic structures illustrated in Figs. 2, 3, 5 and 'i.

In Fig. 12, the windings 59 and 52 again are connected in series but the direction of flow of current in the coil 52 is reversed from the direction illustrated in Fig. 10. Also, in Fig. 13, the coils 5t and 52 are connected in parallel between the terminals 86 and 88 but the directions of flow of current in the coils are similar to those obtained by the connections of Fig. 12. Consequently, the connections of Figs. 12 and 13 are suitable for the magnetic structures illustrated in Figs. 4, 6 and 8.

In Fig. 14, one lead of each of the coils 5D and 52 is connected to the terminal 86. The remaining lead of the coil 59 is connected to the terminal 8B. The remaining lead of the coil 52 is connected to a terminal 92. Consequently, the coils 5E! and 52 may be energized from separate sources of electrical energy and the directions of flow occurring in the two windings depend on the specific connections employed between the sources of energy and the terminals. By properly directing the flow of currents in the two windings, the connections of Fig. 14 may be employed with any of the magnetic structures employed in Figs. 2 to 9, inclusive.

In certain cases, it may be desirable to energize the two windings 5B and 52 from sources which are completely insulated from each other. In such cases, the coils 50 and 52 may have completely independent terminals 86, 94, 88 and 92, as illustrated in Fig. 15. Since the connections to these terminals may be selected to direct current through the windings in any direction, the connections of Fig. 15 may be employed with any of the magnetic structures illustrated in Figs. 2 to 9. For example, the directions of flow of current in the coils 50 and 52 may be selected to apply cumulative torques to the associated shaft of the instrument. The instrument then totalizes the currents supplied to the two coils when the coils are employed with the magnetic structures of Figs. 2 to 8, inclusive. Alternatively, the directions of flow of currents in the two coils may be selected to apply torques differentially to the instrument shaft. The instrument then indicates the diiTerence between the currents energizing the two coils. Also, the two coils 50 and 52 may be employed with the magnetic structure of Fig. 9 to indicate the ratio between two currents. The terminal arrangement of Fig. 15 additionally permits the connection of the coils 50 and 52 externally of the instrument to provide the equivalent of any of the connections illustrated in Figs. 10 to 14. Energization of the coils from independent sources is of particular convenience in some direct-current measuring operations.

The coil connections illustrated in Figs. 12 and 13 are particularly desirable when employed in the instrument of Fig. 8 if astatic operation or freedom from external field influence is desired. In the instrument of Fig. 8, any stray magnetic field which increases the magnetic field strength in one of the air gaps must decrease the magnetic field strength in the remaining air gap. Consequently, such an instrument is substantially astatic in operation.

An instrument designed in accordance with Fig. 7 is shown in detail in Fig. 16. This is an electrodynamic instrument which includes a stator assembly I and a rotor assembly la. The rotor assembly comprises a coil 3 which is mounted for rotation about an axis intermediate two sides 5 and l of the coil. If desired, stub shafts may be attached to each end of the coil 3 for the purpose of mounting the coil for rotation. However, in the specific embodiment illustrated, the coil 3 is secured to a continuous shaft 9 which is mounted for rotation in bearings represented by bearing screws H and I3. The utilization of a continuous shaft simplifies the problem of constructing a sturdy rotor assembly.

In order to connect the coil 3 to an external circuit, a pair of spiral, flexible conductor strips l5 and I! have their inner ends respectively attached to insulating bushings I9 and 2| which are secured to the shaft 9. The terminals or the coil 3 are connected respectively to the inner ends of the conductor strips 15 and IT. The outer ends of the conductor strips are attached respectively to lugs 23 and 25 which are attached to the stator assembly and which are insulated therefrom. Suitable conductors 21 and 29 may be soldered to the lugs 23 and 25 for the purpose of establishing connections between the movable coil 3 and an external circuit.

The rotor assembly is biased towards a predetermined position by means of a spiral control spr'ing 3i Which has its inner end attached to the shaft 9 and its outer end attached to a lever 33 which may be adjusted angularly about the shaft 9 for the purpose of calibrating the instrument. Rotation of the rotor assembly is damped by means of an electroconductive armature disk 35 which is secured to the shaft 9 and which is positioned for rotation between the poles of a permanent magnet 31. Suitable indicating means in the form of an arm 39 is attached to the shaft 9 for rotation therewith. In the specific embodiment of Fig. 16, the arin'iig carries at its end a pen 4| which is positioned for movement across the surface of a chart 43. As well understood in the art, the chart 43 may be advanced continuously with respect to the pen 4! in order to provide a continuous record of the quantity being measured by the instrument.

The stator assembly I includes a magnetic structure 45 which establishes magnetic paths for the magnetic fluxes produced by currents flowing in the coil .3 and in fixed windings and 49 which are associated with the magnetic structure. netic section 5| having a substantially continuous magnetic body or rim portion 53 which substantially surrounds the shaft 8 and the coil 3. Apair of pole pieces '55 and 51 project from opposite interior surfaces of the rim portion 53 to provide arcuate pole faces adjacent the paths of travel of the coil sides 5 and I. In addition the magnetic section 5| has a pair of cantilever or magnetic cores 59 and BI which project from opposite interior faces of the rim portion 53 and which pass through the coil 3 on opposite sides of the shaft 9. These magnetic cores '59 and El are spaced in a direction transverse to the shaft 9 by a distance sufficient to permit passage of the movable coil 3 therebetween. Furthermore, the magnetic cores 5S and 6| have arcuate surfaces spaced from the pole pieces and 5'. to provide a pair of arcuate air gaps within which the coil sides 5 and l are disposed for movement. It will be noted that the magnetic cores 59 and BI provide a substantially cylindrical magnetic core which is attached on opposite sides to the rim portion 53 and which has the passage 53 extending therethroug'h. Since the passage communicates with the air gaps in which the coil sides 5 and l are positioned, the coil 3 may be rotated in a counterclockwise direction (as viewed looking at the rotor assembly from the control spring end) to bring the coil into alignment with the passage 63. The coil then may be moved in a direction parallel to the shaft 9 through the passage 63.

The magnetic section 5| may be formed of any suitable soft magnetic material such as silicon iron. Preferably a material having low hysteresis loss is employed. The magnetic section 5| may be formed of a solid piece of soft iron. However, it is preferable to form the magnetic section 5| of a plurality of laminations as shown in Figs. 16 and 17, particularly if the instrument is designed for measuring alternating current quantities. I he laminations may be provided with suitable openings through which rivets may be passed for the purpose of securing the laminations together. If the magnetic section is formed of l-aminations, the desired contour of each lamination may be accurately formed by a punching operation.

By inspection of Figs. 16 and 17, it will be noted that a separate magnetic path is provided for each The magnetic structure 45 includes .a .ni'ag- 12 of the coil sides 5 and l. The magnetic path for the coil side 5 includes the pole piece 55 and the magnetic core 59 together with the air gap therebetween. The winding 41, when energized, directs magnetic flux through this magnetic path to provide a magnetic field for the magnetic coil side 5. In a similar manner, the magnetic path for the coil side 1 includes the magnetic core GI and the pole piece 5'! together with the air gap therebetween. When the winding 49 is energized, magnetic flux is directed through the associated magnetic path to establish a magnetic field for the coil side 1. Each of the magnetic cores tapers from a large cross section adjacent the rim portion 53 to a smaller cross section adjacent the tip of the magnetic core. Since the magnetic flux varies in magnitude in the direction of the taper, the variation in flux density throughout each of the magnetic cores is not excessive.

Although the magnetic section 5| alone may be employed, some improvement in performance may be obtained by adding thereto an additional magnetic section 51. The reason for this additional magnetic section may be understood by considering the solenoid action of the magnetic section 5| when employed alone. The magnetic section 5| is asymmetric with respect to the path of travel of the movable coil 3. When the coil 3 is in its extreme counterclockwise position (looking at the rotor assembly from the controlspring end thereof) the magnetic reluctance of the magnetic path offered to magnetic flux produced by current flowing through the coil 3 is a maximum. Conversely, when the coil 3 is adjacent its extreme clockwise position the magnetic reluctance oifered to magnetic flux produced by current flowing in the coil 3 is a minimum. Consequently when the coil 3 is energized and the coils 41 and 49 are deenergized, the coil 3 tends to take a position wherein the magnetic reluctance of the associated magnetic path is a minimum. This may be termed a solenoid action and the force applied to the coil 3 by the solenoid action urges the coil in a clockwise direction. In some cases, as when the energization of the fixed windings is constant, it is possible to calibrate the instrument to read correctl despite the presence of this solenoid action. However, this solenoid action is substantially compensated by the provision of the additional magnetic section 61. The compensation permits substantially correct indication by the instrument for substantially all applications thereof.

The magnetic section 51 is similar in construction to the magnetic section 5| but is reversed with respect to the magnetic section 5| about a line transverse to the shaft 9. The magnetic section 61 has a pair of magnetic cores 69 and H which extend through the coil 3 on opposite sides of the shaft 9. In addition, the magnetic section 61 has a pair of pole pieces 13 and 15 which are positioned respectively in the coils 41 and 49. It will be observed that the magnetic cores 69 and 7| are spaced to provide a passage ll therebetween which corresponds to the passage 63 of the magnetic section 5|.

Since the magnetic section 61 is reversed with respect to the magnetic section 5|, the force due to solenoid action which is applied thereby to the coil 3 is opposed to the force developed by the solenoid action of the magnetic section 5|. Consequenly, the resultant magnetic structure is substantially free of errors resulting from solenoid action. The magnetic cores 59, SI, 69 and II all pass through the coil 3 to form a resultant 13 magnetic core therefor which is substantially symmetric with respect to the path of movement of the coil 3.

By inspection of Fig. 16, it will be observed that the passages 63 and 11 are displaced angularly about the shaft 9 with respect to each other. Consequently the coil 3 cannot be removed from the magnetic structure 45 by a simple movement thereof in the direction of the shaft 9. To permit removal of the coil from the magnetic structure the magnetic sections 5| and 31 are spaced from each other along the shaft 9 by a distance 'sufficient to permit movement of a side of the coil 3 therebetween. The desired spacing may be provided by any suitable spacer formed of either magnetic or nonmagnetic material. In the embodiment illustrated in Figs. 16 and 17, the spacer is divided into two parts 19 and 19a. Each of the parts is in the form of a plurality of magnetic laminations which are similar in construction to the adjacent parts of the laminations of the magnetic section 5|. In order to facilitate inspection of the space between the magnetic sections 5| and 61 after assembly thereof, the parts 19 and 19a are located at a substantial distance from each other to provide an opening 192) (Fig. 1'7) in the magnetic structure 45. The space between the magnetic sections is clearly visible through this opening. Consequently, when the magnetic sections 5| and 61 are assembled with the spacer therebetween as shown in Fig. 17, a space is provided between the pair of magnetic cores 59 and 6| and the pair of cores 69 and 1|. This space is sufficient to permit movement of a side of the coil 3 therebetween.

It is believed that the operations required to assemble and disassemble the instrument illustrated in Figs. 16 and. 17 are apparent from the foregoing discussion. To facilitate a further description of such operations, reference will be made to a leading side So of the coil 3 (the lower side of the coil 3 as viewed in Figs. 16 and 17), and a trailing side 31) (the upper side of the coil as viewed in Figs. 16 and 17). It will be understood that the magnetic structure 45 comprising the laminations of the magnetic sections 5| and 61 and the laminations of the spacer is first completely assembled as shown in Fig. 17 wherein a rivet 45a is disclosed for uniting the laminations to each other. Also the rotor assembly I a is completely assembled, the complete assembly including the shaft 9, the coil 3, the conductor strips I5 and H, the disk 35, the arm 39 and the control spring 3|. The rotor assembly then is placed above the magnetic structure 45 (as viewed in Figs. 16 and 17) with the leading side So of the coil aligned with the passage 63 of the magnetic section 5|. The rotor assembly including the coil 3 then is lowered in a direction parallel to the axis 9 to pass the leading side So completely through the passage 63. The leading side 3a. of the coil now is positioned between the pair of magnetic cores 59 and 6| and the pair lowered in a direction parallel to the shaft 9 to pass the leading side 3a completely through the passage 11. The coil now is positioned to embrace the complete resultant magnetic core formed by the magnetic cores 59, 6|, 69 and 1|. The bearing screw II and the support therefor are next placed in position, and the bearing screws II and I3 are adjusted to mount the rotor assembly for rotation with respect to the magnetic structure. The outer ends of the conductor strips I5 and H are soldered to the lugs 23 and 25 and the permanent magnet 31 is positioned as shown in Fig. 16. To complete the installation of the rotor assembly, the outer end of the control spring 3| is soldered or otherwise secured to the lever 33. By following a reverse procedure the rotor assembly Ia may be removed from the magnetic structure 45 without disturbing the magnetic structure in any way.

From the foregoing discussion, it is clear that the magnetic structure 45 is formed of a plurality of unitary laminations each of which has integral pole pieces and magnetic cores. Because of this construction the magnetic structure may .be provided with accurate air gaps, and the accuracy of the air gaps is not disturbed by assembly or disassembly of the instrument.

In certain applications a two-element electrodynamic instrument is required. Such an instrument may be constructed in the manner illustrated in Fig. 18. Referring to Fig. 18, a twoelement electrodynamic instrument is disclosed which includes the two elements 8| and 83. The element 8| comprises a magnetic structure which is similar in construction to the magnetic structure 45 of Figs. 16 and 17. It will be observed that the magnetic structure 85 has associated therewith a pair of fixed windings 81 and 89 which correspond to the fixed windings 41 and 49 of Figs. 16 and 1'7. In addition, the magnetic structure 85 has disposed therein a movable coil 9| which corresponds to the movable coil 3 of Figs. 16 and 17. The element 83 is similar in construction to the element 8| and includes a magnetic structure 93, fixed windings and 91 and a movable coil 99. The magnetic structures 85 and 93 are mounted on suitable supporting posts ||l| and are spaced from each other sufficiently to permit rotation of one of the movable coils therebetween. In certain cases it may be desirable to place magnetic shields I92 between the fixed windings 81 and 95 and between the fixed windings 89 and 91 to prevent magnetic interference between the windings on opposite sides of the shields.

The movable coils 9| and 99 are secured to a common shaft I93 for rotation therewith. This shaft carries a pair of conductor strips I05 for connecting the terminals of the movable coil 99 to an external circuit and a pair of conductor strips I91 for connecting the terminals of the movable coil 9| to an external circuit. These conductor strips correspond to the conductor strips l5 and ll of Fig. 16. In addition, the disk 35, the pen arm 39 and the control spring 3| are secured to the shaft N33 with the disk 35 positioned for movement between the poles of the permanent magnet 31. As well understood in the art, each of the elements 8| and 83 may be energized from a separate pair of conductors of a three-wire circuit or from a separate phase of a polyphase circuit.

Since the principles employed in the construction of the instrument illustrated in Figs. 16 and 1'7 are also employed for the instrument of Fig.

15 18, it follows that the rotor assembly in Fig. 18 may be introduced into operativ position with respect to the magnetic structures 85 and 93 or may be removed therefrom without disturbing the magnetic structures in any way. For example, in constructing the instrument, the magnetic structures 85 and 93 are completed and are secured to the supporting posts lfll. For convenience in discussing the assembly of the instrument, the coil 99 will be referred to as having a leading side 99a and a trailing side 9%. The coil 9! will be referred to as having a leading side em and a trailing side 911). This corresponds to the notation employed for the coil 3 of Figs. 16 and 17. The rotor assembly of Fig. 18 is first completely assembled. This rotor assembly includes the shaft I93, the coils 9i and 99, the conductor strips I95 and I07, the disk 35, the pen arm 39 and the control spring 31. The rotor assembly then is placed above the magnetic structure 85 as viewed in Fig. 18 with the leading side 99a. of the coil 99 positioned above the adjacent passage in the magnetic structure 85. The rotor assembly then is dropped in a direction parallel to the shaft !03 rotated and again dropped to position the coil 99 for embracing the magnetic cores of the magnetic structure 85. This procedure is exactly similar to that employed for dropping the coil 3 of Figs. 16 and 17 through the passages 63 and Ti to embrace the associated magnetic cores.

The coil 99 then is passed completely through the magnetic structure 85 by rotating the coil until its trailing side 9911 is in position to drop through the adjacent passage in the magnetic structure 85. After the trailing side has passed through the adjacent passage the coil 99 is rotated to pass the trailing side 99?) between the magnetic sections of the magnetic structure 85 until the trailing side 99?) is positioned to drop through the lower passage in the magnetic structure. The coil 99 now is lowered to a position between the magnetic structures 85 and 93.

The operation of passing a coil completely through its magnetic structure may be understood more fully by a further consideration of Figs. 16 and 17. Assuming that the coil 3 is in the position illustrated in Figs. 16 and 17 and that it is desired to drop the coil completely through its associated structure 45, the coil is rotated until its trailing side 31) is adjacent the passage 63 in the magnetic section 5!. The coil now is lowered until the trailing side 31) is psitioned between the magnetic sections and 67. By suitably rotating the coil 3 in a clockwise direction (looking at the rotor assembly from the control spring end thereof) the trailing side 3b is moved through the magnetic sections BI and 31 to a position wherein the trailing side is in alignment with the passage T! in the magnetic section 91. The trailing side 317 now may be dropped through the passage 1! to complete the passage of the coil 3 through its associated magnetic structure 45. The operation of passing the coil 99 of Fig. 18 completely through the magnetic structure 85 is similar to that discussed for the coil 3 of Figs. 16 and 1'7.

With the coil 99 positioned between the magnetic structures 85 and 93 of Fig. 18, the rotor assembly is rotated to bring the leading side 9%: of the coil 99 into alignment with the adjacent passage of the magnetic structure 93. The coil 99 next is dropped, rotated and again dropped in the manner previously discussed with reference to the coil 3 of Figs. 16 and 17 until the Cir coil 99 is in position to embrace the magnetic cores of the associated magnetic structure 93. Since the magnetic structures and 93 are similar, the movement of the coil 99 from a position between the magnetic structures 95 and 99 to a position wherein the coil 99 embraces the magnetic cores of the magnetic structures 93 also moves the coil 9| from a position above the magnetic structure 85 to a position wherein the coil 9! embraces the magnetic cores of the associated magnetic structure 85. Consequenth both of the coils 9| and 99 are in their operative positions with respect to their associated magnetic structures. With the rotor assembly of Fig. 18 in this posiiton, the bearings associated with the shaft I03 may be adjusted and the outer ends of the conductor strips and I9! may be connected as discussed with reference to Figs. 16 and 17. In addition, the outer end of the control spring 3| may be connected to its associated lever 33 and the permanent magnet 31 may be moved to operative position with respect to the disk 35. The instrument of Fig. 18 now is in completely assembled condition. By following a reverse procedure, the rotor assembly of Fig. 18 may be removed completely from the magnetic structures 85 and 93 without disturbing the magnetic structures in any way.

Referring again to Figs. 16 and 17, the windings 41 and 49 are connected in series and are so energized that if direct current is passed therethrough, magnetic flux flows through the pole pieces in the directions illustrated by the arrows 99 and I IQ of Fig. 15. If the windings are energized by alternating current, the arrows I99 and H0 represent instantaneous directions of magnetic flux flow.

The connections of electrodynamic instruments to external circuits are well understood in the art. Suitable connections for the instrument illustrated in Figs. 16 and 17 are illustrated in Fig. 19 wherein the instrument is associated with a single-phase alternating-current circuit having conductors LI and L2. If the instrument is to be employed for measuring power, the movable coil may be energized in accordance with current flowing in the associated electrical circuit whereas the fixed windings may be energized in accordance with the voltage of the associated electrical circuit. However, as a rule, the movable coil is energized in accordance with voltage. As illustrated in Fig. 19, the coil 3 is connected across the conductors LI and L2 through suitable multiplier resistors Ill and H3. The windings 41' and 49 are connected in series in one of the conductors LI for energization in accordance with current flowing in the associated electrical circuit.

In instruments of the type herein disclosed which are provided with iron cores, and which. are employed for measuring alternating quantities, some compensation is desirable for the hysteresis losses in the iron core. The losses may cause the current and magnetic flux lag in the current winding circuit to be less or greater than the inductive lag in the voltage coil circuit. Such compensation is provided in Fig. 19 by a capaci tor I I5. Depending on the magnitude and direction of the correction required, the capacitor l 15 may be connected across the coil 3*, across the coil and a part of the multiplier resistance (if the iron losses cause a flux lag in the current circuit greater than the inductive lag in the voltage coil circuit), or across the multiplier resistor alone (if the iron losses cause a flux lag in the current winding circuit less than the inductive lag in the voltage coil circuit). In a specific instrument designed in accordance with the invention, it was found that suincient compensation was obtained by connecting the capacitor I I across one-half the total multiplier resistance as illustrated in Fig. 19. The value of capacitance and the proportion of the resistance shunted thereby are selected to equalize the aforesaid lags.

One of the problems presented by the instrument of Fig. 16 resides in the provision of adequate insulation for the fixed windings 41 and 49. A desirable form of insulation which may be employed for the fixed windings is illustrated in Figs. 20 and 21 and is shown in Fig. 20 associated with the fixed winding 41. The insulation is formed of two'U-shaped parts I25 and I21 which are similar in construction but which are inserted from opposite faces of the magnetic structure 45. The U-shaped part I25, for example, has a channel I25e between the flanges of which a part of the winding is disposed. From the ends of the channel web two generally tubular legs I25a and I25b project and surround portions of the fixed winding 41. In a similar manner the part I21 has generally tubular legs I21a and I21b. It will be noted that the leg I21a has a continuous male lip I210 projecting therefrom which is designed for reception in a female seat I250 formed in the tubular leg I25a. In a somewhat similar manner, the leg I25b has a continuous male lip I25d designed for reception in a female seat I21d formed in the tubular leg I21b. Because of the overlapping or telescoping relationship of the legs on the parts I25 and I21, adequate insulation is provided for the winding 41 throughout its passage through the magnetic structure 45. The exposed ends of the parts I25 and I21 are open to facilitate the winding of the coil 41 therein. However,

as clearly shown in the drawings, the parts I-25 and I21 are configured to introduce adequate insulation between the fixed winding 41 and adjacent portions of the magnetic structure 41. The parts I25 and I21 conveniently may be molded from an insulating material, such as a phenolic resin.

In Fig. 22, a complete two-element electrodynamic instrument is disclosed which is extremely easy to assemble and service. In this instrument a supporting structure I29 is divided into two separable parts I29a and I29b. These parts are attached in any suitable manner as by means of machine screws I3I, one of which is shown in Fig. 22. It will be noted that the part I29 has an extension I33 overlapping a portion of the part I290. and that the extension I33 is provided with a separate opening I35 through which each of the machine screws I3I passes.- By removing the machine screws I3I, the part I29a may be detached from the part I292). The supporting structure I29is secured to a mount I290 which may be in the form of a panel by means of insulating posts I29d which may be formed of a phenolic resin. Each post has a stud I29e in threaded engagement with the supporting structure I29 and a stud I29f'suitablysecured to the mount I290, as by a nut I299. The studs I29e and I29f are insulated from each other. Consequently, the entire instrument is insulated from the mount I290.

The parts I29aand I29b have, respectively, arms I31a and I311) which have-secured thereto internally-threaded bushings I39a and I39b in any suitable manner as by means of machine 18 i screws I4I. Bearing screws Ba and 3b are in threaded engagement respectively with the bushings I39a and I391).

The bearing screws are employed for mounting a rotor assembly I45 for rotation with respect to the supporting structure. The rotor assembly includes a shaft I41 having an opening I49 at one end for receiving a bearing I5I. The bearing screw I 43a has a pin I53 projecting therefrom for reception in the bearing I5I. At its remaining end, the shaft I41 has projecting therefrom a pivot I54 which is received in a suitable bearing mounted in the bearing screw I43b.

The rotor assembly I45 is provided with two coil assemblies I55 and I51. These coil assemblies form portions respectively of the two elements I59 and I 6| of the electro-dynamic instrument. Although the coil assemblies I55 and I51 each may be similar to the coil 3 of Figs. 16 and 17, for the reasons hereinafter set forth each of the coil assemblies I55 and I51 employs the two coils 50 and 52 which have been previously discussed. The coils 50 and 52 may be secured to the shaft I41 for rotation therewith in any suitable manner. For example, the coil assembly I55 includes two channels I63 and I65 which have secured thereto bushings I63a and 5511. These bushings are firmly secured to the shaft I 41 as by establishing a press fit therebetween. Although the bushings may be constructed of metal, if additional insulation between the coils is desired, the bushings may be constructed from insulating material such as a phenolic resin. The channels I63 and I65 are proportioned to receive portions of the ends of the coils 50 and 52 and are bent around the coils to secure the coils firmly in position. If desired, adhesive may be employed between the coils 50 and 52 and their associated channels I63 and I65.

The shaft I41 also has secured thereto a collar I61 to which a spring arm I69. is attached. The collar I61 may be secured in position by means of a set screw I1I. A damping disk or armature I13 is mounted on the shaft I41 to form a part of the rotor assembly. This armature may be formed of a suitable electroconductive material, such as aluminum or copper.

In order to establish connections between external circuits and the coils 50 and 52, a terminal assembly I15 is secured to the shaft I41. To facilitate construction and servicing operations, the terminal assembly is mounted on a metallic sleeve I11 which has a shoulder I19 at one end thereof. This shoulder has associated therewith a set screw I8I by means of which the entire terminal assembly is detachably secured to the shaft I41. If desired, the shaft I41 may have a groove I83 formed therein for reception of the set screw I8I. This groove eliminates difficulty in removal of parts due to burrs formed on the shaft by the set screw.

The terminal assembly I15 includes six collars I85 which surround the sleeve I11 and which are well nested within each other. For example, the top collar I85 has a recess I 85a within which a lip I851) formed on the adjacent collar projects. By this construction, the collars form a complete layer of insulation over the sleeve I11. Conveniently, the collars may be molded from an insulating material such as a phenolic resin. It will be noted that the shoulder I19 carries a pin I81 which is received in a notch in the adjacent collar I85 to locate the collar angularly with respect to the sleeve I11.

A separate mica barrier I89 is positioned between each pair of collars I85. Between each pair of mica barriers a separate flexible spiral conductor strip I9], I93, E35 or it? is positioned. Each of these conductor strips has its inner end attached to a holder which also is secured be tween an adjacent pair of collars I35. For example, the inner end of the conductor strip iti is soldered to a lug I39 projecting from a holder This holder, together with the adjacent mica barrier, is securely clamped between the two upper collars The holder 2% also has a lead passing through an insulating tube 263 for establishing a connection to a lead 285 from one of the coils. In a similar manner, the remaining conductor strips are attached to other coil leads.

The outer end of each conductor strip soldered to a lug which is mounted on an insulation block 20?. The insulation block is secured in any suitable manner to the part l29b of the supporting structure. For example, the outer end of the conductor strip [9! is soldered to a lug 191a. In a similar manner, the outer ends of the conductor strip H33, H and I9! are connected respectively to lugs i93a, H350. and lQic. The lugs, in turn, may be connected to any external circuit as desired.

In constructing the terminal assembly, the collars, mica barriers, conductor strips and holders are successively slid over the sleeve It? in the order indicated in Fig. 22. The upper end of the sleeve I11, as viewed in Fig. 22, then is flared or spun over the upper collar to secure the parts of the terminal assembly to each other. Thereafter, the terminal assembly may be handled as a unit for attachment or. removal with respect to the shaft I47. The relationship of the collars, barriers, conductor strips and holders may be substantially similar to that described in the copending application of D. A. Young et al. Serial No. 500,896, filed September 2, 1943, which issued as Patent 2,438,027 on March 16, 1948.

A collar 299 is removably attached to the shaft I47 by any suitable means, such as a removable pin 2H which passes through the shaft and collar. This collar may support a pen arm 2l3. It shouldbe noted that to remove the pen arm, it is only necessary to remove the machine screw l3l, thereby permitting the part I29a to be separated from the part I29?) of the supporting structure. The bearing 15! and the collar 2339 then may be removed from the shaft. This servicing operation does not necessitate any cutting or unsoldering of electrical connections.

The coil assemblies I55 and I5! have associated therewith magnetic structures 2 l5 and 2 I! which may be similar in construction to the magnetic structure of Fi s. 16 and 17. However, in Fig. 22 the insulating parts I25 and I2? are illustrated for the fixed windings associated with the magnetic structures 215 and M1. The magnetic structures 2l5 and 21'! are secured in any suitable manner to a bracket 2 l9 projecting from the part I292) as by means of machine screws (not shown). Desirably, the magnetic structures may be electrically insulated from the bracket 2 l9, as represented by insulation 2 I 90:. A suitable structure for this purpose is shown in the aforesaid Young et al. application. The air gaps in which the coils are located may provide adequate insulation between the coil assemblies I and I51, but in some cases the additional insulation 2190. may be desirable in the path represented by the magnetic structures and the bracket.

Dampin flux for the armature H3 is provided by a pair of permanent magnets 221 and 223 which are spaced to define an air gap through which a portion of the armature I13 rotates. The damping'magnets are secured to a post 225 which has a stud 221 projecting through an opening :in a bracket 229 secured to the part l29b of the supporting structure. A nut 228 cooperates with the stud 221 to secure the post 225 to the bracket 229.

The construction of the damping magnet assembly is shown more clearly in Fig. 23. Referring to Fig. 23, it will be noted that the permanent magnets 22! and 223 are suitably secured to a plate 23! of nonmagnetic material, such as brass. For example, the permanent magnets may be brazed to the plate 23 l. The polarities of the permanent magnets may be those indicated in Fig. 23 by the polarity markings N (north pole) and S (south pole). It will be noted that the plate 23! has an opening 233 through which a portion of the armature 113 projects (see Fig. 22).

As shown in Fig. 23, the post 225 has a stud 235 projecting therefrom through an opening in the plate 23!. This stud 235 has nuts 23'! and 239 positioned on opposite sides of the plate 23!. By proper manipulation of the nuts 23! and 239, the plate-23! may be secured to the stud 235 at various positions in order to adjust the distance between the permanent magnets and the shaft 141. Such adjustment is effective for adjusting the damping torque applied by the permanent magnets to the rotor assembly. Additional support for the plate 23l is provided by the sleeve 24] which is secured to the plate 231 and which extends in a direction parallel to the stud 235. This sleeve 24] is proportioned to receive slidably a guide rod 243 which is secured to the bracket 225. It should be noted that by release of the nut 239, the plate 23! may be removed completely from the remainder of the instrument.

To remove the rotor assembly from the remainder of the instrument illustrated in Fig. 22, the set screw I'H may be released to permit the shaft M1 to pass through the collar I61 during subsequent operations. The machine screws l3l then are withdrawn and the part 129a of the supporting structure is removed from the part [292). If desired at this stage, the bearing 15! and the collar 209 may be removed from the shaft M1. The nut 239 also is removed from its associated stud 235 and the plate 23!, together with the permanent magnets mounted thereon, is removed from the remainder of the instrument.

The outer ends of the conductor strips l9l, I93, I95 and I9! are next detached from their associated lugs l9la, I93a, l95a and I9'la. This completely frees the rotor assembly for removal from the remainder of the instrument. Since the magnetic structures 2I5 and 2| 1 are similar to the magnetic structure 45 of Figs. 16 and 17 and the magnetic structures and 93 of Fig. 18, they provide continuous passages through which the coil assemblies may be moved during withdrawal of the rotor assembly from the remainder of the instrument. This withdrawal is effected in exactly the same manner required for the rotor assembly of Fig. 18. For this reason, a detailed discussion of the angular and axial movements of the rotor assembly during withdrawal is believed unnecessary at this time. It is to be understood that the bracket 2H! provides adequate clearance for rotation of the coil assemblies during their withdrawal from their associated ma netic struc- 21 tures. The reverse of the above procedure may be followed for reassembling the instrument.

The arm I69 may be clamped to the outer end of a spiral control spring 241 in any suitable manner. As shown in Fig. 22, the arm I69 has has a portion 249 extending parallel to the shaft I41. The portion 249 has an opening through which a machine screw 25! passes into threaded engagement with a strip 253. The strip 253 extends through an opening 255 in the arm I69. By manipulation of the screw 25l, the outer end of the control spring 241 may be detachably clamped between the portion 249 and the strip 253, as shown in Fig. 22.

The inner end Of the spiral control spring 241 is secured in any suitable manner to a holder 251 which is secured to the bushing l39b. It will be noted that the bushing [391) has a groove formed therein to provide a section 259 of reduced diameter. The holder 251, as more clearly shown in Fig. 24, has a recess 26! formed in its lower face. This recess has a diameter substantially equal to the diameter of the bushing 13% below the portion 259. In addition, the holder 251 has a channel 263 out therein. This channel has a width substantially equal to the diameter of the section 259 of the bushing. Furthermore, the holder 251 has 'a thickness slightly less than the thickness of the groove provided in the bushing l39b. To remove the holder 251 from the bushing I391), the holder is raised slightly as viewed in Fig. 22. The holder then may be withdrawn to the right, completelyclearof thebushing. To reinsert the holder in mounted position, a reverse procedure is followed. The holder 251 is urged in a downward direction by means of a spring washer 265 which biases in a downward direction a. washer 261. The washer 261 engages the holder 251 to urge the lower portion of the bushing 1391) into the recess 26!. If desired, a handle 269 may be attached to the washer 261 for the purpose of rotating the holder 251. The engaging surfaces of the washer 261 and the holder 251 may be roughened or knurled to assure firm engagement therebetweenduring rotation of the washer 291. Manipulation of the handle 269 may be employed for adjusting the rotor assembly to the position it should occupy when the instrument is deenergized. The holder 251-may be placed in its approximate correct position by manual rotation thereof with respect to the washer 291. It should be noted that by releasing the screw 25 l, the control spring 241 may be removed for repair or replacement without disturbing the remainder of the instrument.

The provision of two coils 50 and 52 in each of the elements provides anextremely flexible instrument. -For example, the coils in each of the elements may be connected in accordance with the teachings, of Fig. 12 and the f xed windings in each of the elements then may be connected inseries in accordance with the teachings of Fig. 8. As previously explained, the resulting instrument is substantially astatic in operation.

Although Figs. 16, 1'1, 18 and 22 illustrate preferred embodiments of the invention, it is possible to connect sets of pole pieces embodying the invention in other practical ways. For example, in Fig. 25 an electrodynamic instrument is disclosed which corresponds to the instrument illustrated schematically in Fig. 3. In Fig. 25 a pair of outer pole pieces 215 and 211 are disclosed which correspond to:the pole pieces 4 and 6 of Fig. 3. In addition, a pair ofinner pole pieces 219 and .28l aredisclosed'which correspond to the inner pole pieces 38 and 40 of Fig. 3. The outer pole pieces 215 and 211 are connected by a magnetic conductor 283 which corresponds to the magnetic conductor [2 of Fig. 3. The inner pole pieces 219 and 28! are connected by a magnetic conductor 285 which corresponds to the magnetic conductor 42 of Fig. 3.

When the winding l4 which surrounds the magnetic conductor 283 in Fig. 25 is energized toproduce a flow of magnetic flux as illustrated by the arrow the magnetic flux flows from the pole piece 211 through the air gap to the inner pole piece 28!, through the magnetic conductor 285 and the pole piece 219, across the air gap to the outer pole piece 215 and through the magnetic conductor 283 back to the pole piece 211.

The inner pole pieces 219 and 28 I, together with the magnetic conductor 285 form a magnetic section 281 which, if employed alone, produces a substantial solenoid action with respect to the coil l9 associated therewith. To eliminate substantially this solenoid action, a second magnetic section 289 is employed which is similar in construction to the magnetic section 281. However, the magnetic section 289 is reversed relative to the magnetic section 281 about an axis transverse to the axis of rotation of the coil It. For the reasons set forth in the discussion of Fig. 16, the provision of the two sections 281 and 289 substantially eliminates solenoid action.

Conveniently, the outer pole pieces 215 and 211, together with the magnetic conductor 283 may be machined from a suitable soft magnetic material such as soft steel. The magnetic sections 281 and 289 may be formed of a plurality of laminations which are attached to each other in any suitable manner as by means of rivets 29 I. The magnetic sections 281 and 289 are spaced apart by means of a suitable spacer 293 which may be formed of magnetic or nonmagnetic material, as desired. The sections 281 and 289, together with the spacer 293, may be riveted to a supporting plate 295 of nonmagnetic material such as brass which, in turn, is suitably attached to the pole piece 215 as by brazing. Since the magnetic sections 291 and 289 are spaced apart by a distance sufiicient to permit rotation of a side of the coil [6 therebetween, the coil 16, together with its shaft, may be removed from the magnetic structure of Fig. 25 without disturbing the magnetic structure in any way. The procedure for removing the coil i6 is similar to that employed for removing the coil 3 from the magnetic structure illustrated in Figs. 16 and 1'1. For this reason, a further discussion of such removal is believed to be unnecessary.

In Fig. 26, an electrodynamic instrument is disclosed which corresponds to the instrument schematically shown in Fig. 4. As shown in Fig. 26, a pair of outer pole pieces 291 and 299 correspond. to the pole pieces 4 and 5 of Fig. 4, and form part of an upper magnetic section 399. A pair of inner pole pieces 39! and 363 correspond to the pole pieces 38 and 4-8 of Fig. 4. The outer pole piece 291 is connected to the inner pole piece 303 by means of a magnetic conductor 395. The outer pole piece 299 is connected to the inner pole piece 38! by means of a magnetic conductor 391. When the winding i4 is energized to produce magnetic flux in the direction represented by the arrow (11, the flux flows in series through the magnetic conductor 395, the inner pole piece 393, the outer pole piece 299, the inner pole piece 31H and the outer pole piece 291, which constitute parts of the upper magnetic section 399. Because oi'the-idirections of flow of flux, the two coils 59 and 52*are employed in the modification of Fig. 26. In order to eliminate solenoid action, a second magnetic section 3!! is provided which is similar in construction to the upper magnetic section'3fl9. However, the magnetic section 3H is reversed relative to the magnetic section 369 about an axis transverse to the axis of rotation of the coils 59 and 52. In Fig. 26,"the coil :4 surrounds both of the magnetic conductors sociated with the two magnetic sections. Conveniently, in each magnetic section the pole pieces 291 and 333, together with the magnetic conductor 335 .may be formed of integral magnetic laminations of soft iron which are attached to each other by means of rivets 3! 5. In a simiiar man nor, in each magnetic section the pole ies and 39!, together with the magnetic conductor 301', may be formed of soft magnetic laminaticns which are united by means of rivets 3 i i The two magnetic sections 359 and 3!! may be secured in any suitable way to a nonmagnetic support i By inspection of Fig. 26, it will be noted that the magnetic sections 369 and 3H are spaced apart by a distance sufiicient to permit rotation of a side of each of the coils and 52 therebetween. Consequently, the coils 58 and together with their shaft, may be removed from the magnetic sections without disturbing the magnetic sections in any way. The procedure for removing the coils is similar to that discussed for Figs. 16 and 1'7.

Fig. 27 shows an electrodynamic instrument which corresponds to the instrument shown schematically in Fig. 5. In Fig. 27, an upper mag netic section 321 includes an inner magnetic pole piece 3"!9 and an outer pole piece coth con nected to one end of a magnetic conductor 32!. An inner magnetic pole piece 323 and an outer magnetic pole piece 325 are connected to the maining end of the magnetic conductor i which corresponds to the magnetic conductor of Fig. 5. The pole pieces 3l9, 326, 323 and 325 of Fig. 27 correspond to the pole pieces 43, 5, and 6 of Fig. 5. When the coil !4, which surrounds the magnetic conductor 32!, is energized to produce a flow of magnetic flux in the direction represented by the arrow the flux passes from the pole pieces 323 and 325, respectively, to the pole pieces 320 and 3H9. As explained with reference to Fig. 5, the coil 45 may be employed in an instrument of the type illustrated in Fig. 27.

The upper magnetic section 321 which is composed of the pole pieces 3E9, 323, and together with the magnetic conductor 3H, conveniently may be formed of a plurality of integral magnetic laminations of soft iron which are attached to each other by means of rivets 329. To eliminate the solenoid action produced by the magnetic section 321, an additional magnetic section 33! is employed which is similar to the magnetic section 321. However, the lower magnet c section 33! is reversed with respect to the magnetic section 321 about an axis transverse to the axis of rotation of the coil It. It will be noted that the coil !4 surrounds the magnetic con-- ductors 32! of both of the magnetic sections.

A spacer 333 of magnetic or nonmagentic ma terial is interposed between the magnetic sections 321 and 33! to space them apart for a distance suiiicient to permit rotation of a side of the coil I6 therebetween. By inspection of Fig. 27, it will be observed that the coil !6 can be removed from the associated magnetic sections without disturbing the'magnetic sections in any way by 24 following the procedure discussed with re to Figs. 16 and 1'1. It is to be the coils !5 of Figs. 25 and 2"! magnetic sections.

In Fig. 28, an electrodynamic instruinc closed. which corresponds to the schematically shown in Fig. 6. The instru I of Fig. 28 includes a pair of oute? magnetic pole pieces 335 and 331 which corres ond to the poie pieces 4 and 6 of Fig. 6. In addition, Fig. 28 shows an upper magnetic section 3% providing a pair of inner magnetic pole pieces and 3-3. which correspond to the inner pole pie-ties and of Fig. 6. The inner magnetic po-ie pieces and 34! are connected to one end a conductor 343 which corresponds to th conductor 58 of Fig. 6. The outer maul ctic poie pieces 335 and 331 are connected to the remain-- ing end of the magnetic conductor 343. The coils 56 and 52 in Fig. 28 are attached to a shalt which also carries flexible spiral conductor strips 341 and 349 and a spiral control spring 35 i. A pointer Or pen arm 353 also may be mounted on the shaft 345.

The outer magnetic pole pieces 335 and 331 are connected by a magnetic conductor 355 which corresponds to the magnetic conductors 14 and 16 of Fig. 6. The remaining ends of the pole pieces 335 and 331 may be connected by a bridging plate 351 of magnetic or nonmagnetic material which is detachably secured to the pole pieces by means or" machine screws 359. The plate 351 and the magnetic conductor 355 have threaded openings passing therethrough for receiving bearing screws 36! and 353.

The inner ends of the conductor strips 341 and 349 are secured to insulating bushings mounted ontlie shaft 345. They are also connected to the coils 50 and 52 in a manner such as that illustrated in Fig. 12. The outer ends of the conductor strips may be secured to the outer pole pieces in any suitable manner. For example, the outer end of the conductor strips 349 is secured to an insulating block 365 which, in turn, is secured to the pole piece 335. The outer end of the control spring 35! may be connected to a post 351 which, in turn, is secured to the pole piece structure in any suitable manner. When the coil M which surrounds the magnetic conductor 343 is energized to produce a direction of flux how similar to that represented by the arrows 1, mag netic flux flows from the magnetic conductor 343 in parallel through the air gaps between the inner and outer pole pieces. The inner pole pieces 339 and 34!, together with magnetic conductor 343, forming the upper magnetic section 359 may be constructed from a, plurality of laminations of soft magnetic material such as soft iron which are attached to each other by means of rivets 3Y9.

In order to eliminate solenoid action, a second magnetic section 31! is provided which is similar in construction to the magnetic section 369. The magnetic section 31! is reversed with respect to the magnetic section 369 about an axis transverse to the shaft 345 and parallel to the ma netic conductor 343. The magnetic sections 365 and 31! are spaced apart by means or" a spacer 313 for a distance sufiicient to permit rotation of a side of each of the coils 59 and 52 therebetween. It is to be understood that the coil !4 surrounds the magnetic conductors 343 of both of the magnetic sections 369 and 31!.

In order to remove the coils 50 and 52 from the associated magnetic structure, the ends of the conductor strips 341 and 349 are released from their associated pole pieces. In addition, the outer end of the spiral control spring 35I is detached from the post 361. Next the screws 359 are removed and the bridge plate 351 also is removed from the pole pieces.

The rotor assembly now is rotated in a counter-clockwise direction (looking at the controlspring end of the shaft 345) until the coils 50 and 52 are aligned with the passage between the inner pole pieces of the magnetic section 31!. The entire rotor assembly then is raised until the lower ends of the coils 50 and 52 are adjacent the magnetic section 369. The rotor assembly next is rotated in a clockwise direction until the coils 50 and 52 are aligned with the passage between the inner pole pieces of the magnetic section 369. During this rotation the lower ends of the coils rotate between the magnetic sections 369 and 3H Finally the rotor assembly is raised to pass the coils through the passage'between the inner pole pieces of the magnetic section 369. The rotor assembly now is completely free of its associated magnetic structure. By following a reverse procedure, the instrument may be reassembled.

Although the invention has been described with reference to certain specific embodiments thereof, numerous modifications are possible. Therefore, the invention is to be restricted only by the appended claims as interpreted in view of the prior art.

I claim as my invention:

1. In an electrical instrument, a magnetic structure having an air gap, a coil positioned in said air gap, and means mounting said coil for rotation relative to said magnetic structure, said magnetic structure extending through said coil to establish magnetic paths for directing magnetic flux through two sides of the coil, and said magnetic structure being configured to provide a, passage through which said coil may be withdrawn from operative position relative to the magnetic structure to a 'position external to said magnetic structure, said magnetic structure including a pair of magnetic-fiux-producing means, each independently effective for directing magnetic flux through a separate one of the coil sides.

2. In an electrical instrument, a unitary magnetic structure, coil means, and means mounting said coil means for rotation about an axis rela- ,tive to said magnetic structure, said coil means comprising two coils angularly spaced about the axis of rotation, the coils respectively having two sides spaced substantially from the axis of rotation thereof, saidmagnetic structure comprising portions extending through said coils to define with the remainder ofsaid magnetic structure a separate air gap for each ofsaid two sides of said coils, and a separate source of magneto motive force for establishing a separate magnetic field in each of the air gaps.

3. In an electrical instrument, a magnetic structure, a coil, a continuous shaft supporting said coil for rotation about the axis of the shaft, and means mounting said coil and shaft for rotation relative to said magnetic structure, said coil having two sides spaced substantially from the axis of rotation thereof, said magnetic structure comprising portions extending through said coil on opposite sides of said shaft to define with theremainder of saidmagnetic structure a separate air gap for each of said two sides of said coil, the magnetic structure including two magnetic-flux-producing means each independently effective for directing magnetic flux through a separate one of the air gaps.

4. In an electrical instrument, a magnetic structure, coil means, and means mounting said coil means for rotation relative to said magnetic structure, the coil means comprising two coils angularly spaced about the axis of rotation, said magnetic structure comprising a first magnetic portion defining a first magnetic path having an air gap within which a side of a first one of said soils is positioned for movement, and a second magnetic portion defining a second magnetic path having an air gap Within which a side of a second one of said coils is positioned for movement, said portions being spaced to define a passage communicating with said air gaps through which said coil may be removed from operative position relative to said magnetic structure, the magnetic structure including magnetic-fiux-producing means for directing magnetic fiux in opposite directions through the air gaps.

5. In an electrical instrument, coil means, a magnetic structure, means mounting said coil means for rotation relative to said magnetic structure about an axis, said coil means including a pair of coil sides spaced angularly about said axis and positioned substantially at the same distance from any point on said axis, said magnetic structure having a separate air gap for receiving each of said coil sides, and the magnetic structure having a passage extending between said air gaps through the position occupied by said axis, and separate magnetomotive-force-producing means independently effective for establishing a separate magnetic field in each of said air gaps, the total value of the magnetic flux in the two magnetic fields being substantially equal to the total magnetic flux produced by the separate means, said passage being proportioned to permit movement therethrough of said coil means from a position external to said magnetic structure in a substantially linear direction to a position wherein said coil sides may be rotated substantially about said axis through said air gaps.

6. In an electrical instrument, coil means, a magnetic structure, means mounting said coil means for rotation relative to said magnetic structure about an axis, said coil means including a pair of coil sides substantially equidistant from one end of the coil means and spaced angularly about the axis, said magnetic structure including first and second magnetic inner pole pieces disposed between the coil sides, first magnetic means spaced from said first magnetic inner pole piece to define therewith a first air gap Within which a first one of the coil sides is disposed for rotation, second magnetic means spaced from said second magnetic inner pole piece to define therewith a second air gap within which a second one of the coil sides is disposed for rotation, the coil means being rotatable from a first position wherein said coil sides are disposed within their respective air gaps to an intermediate position wherein said coil sides are external to the air gaps, said inner pole pieces being spaced to define a passage permitting movement therethrough of said coil means from the intermediate position to a position external to said magnetic structure, and magnetic means extending between said inner pole pieces by a path external to said air gaps and said passage,

said magnetic structure including two magnetic-' 27 flux-producing means for producing magnetic fields in said air gaps, each of the magnetic-fluxproducing means being effective when independently energized for establishing a magnetic field in a separate one of the air gaps.

7. In an electrical instrument, coil means, a magnetic structure, means mounting said coil means for rotation relative to said magnetic structure about an axis, said coil means including a pair of coil sides spaced angularly about said axis and positioned substantially at the same distance from any point on said axis, said magnetic structure having a separate air gap for receiving each of said coil sides, and means for directing magnetic flux in opposite directions through said air gaps, said coil means including means connecting said coil sides for energization in directions conferring substantial immunity against external field influence on said instrument.

8. In an electrical instrument, coil means, a magnetic structure, means mounting said coil means for rotation about an axis relative to said magnetic structure, said coil means comprising a pair oi coils spaced angularly about said axis and positioned substantially at the same distance from any point on the axis, said magnetic structure having a separate air gap for each of said coils, and means for directing magnetic flux in opposite directions through said air gaps, and means connecting said coils for energization from a single source of electrical energy to apply cumulative torques to said shaft, whereby substantial immunity against external field influence is conferred on said instrument.

9. In an electrical instrument, coil means, a magnetic structure, means mounting said coil means for rotation about an axis relative to said magnetic structure, said coil means comprising a pair of coils spaced angularly about said axis and positioned substantially at the same distance from any point on the axis, said magnetic structure having a separate air gap for each of said coils, and separate means for directing magnetic flux through each of said air gaps.

10. In an electrical instrument, a magnetic structure, a pair of windings, coil means, means mounting said coil means for rotation about an axis with respect to said magnetic structure, said magnetic structure comprising a pair of pole pieces positioned on opposite sides of said axis, each of said pole pieces projecting through a separate one of the windings and having a pole face adjacent the path of travel of a separate side of said coil means, a pair of magnetic cores projecting through said coil means on opposite sides of said axis, and a magnetic body connecting said pole pieces to one end of each of said magnetic cores, said ends of said magnetic cores being disposed on opposite sides of said axis, whereby each of said pole pieces cooperates with a separate one of said magnetic cores to complete a magnetic path having an air gap within which a separate side of said coil means is disposed for movement, said magnetic cores being spaced in a direction transverse to said axis to define a passage through which said coil means may be removed from operative position relative to said magnetic structure, each of said windings when independently energized cooperating with the pole piece therein to direct substantially the entire magnetic flux produced by the winding through the pole face of the pole piece therein.

11. In an electrical instrument, a magnetic structure including a substantially cylindrical magnetic core, a magnetic body substantially surrounding saidmagnetio core, said magnetic body having a pair of pole pieces spaced from said magnetic core andspaced angularly about the axis of said magnetic coretoform therewith a pair of susbtantially annular air gaps, said magnetic core having a passage extending completely therethrough intermediate said air gaps to divide said magnetic core into two portions each adjacent a separate one of said pole pieces, said magnetic structure including separate magnetic means displaced from said air gaps for connecting each of said magnetic portionsto said magnetic body to define a pairv of magnetic paths each including a separate one of said pole pieces and a separate one of said magnetic portions, coil means proportioned for movement from a position external to saidmagnetic structure through said passage to a position embracing said magnetic core with coil sides located in said air gaps, means mounting said coil means for rotation relative to said magnetic structure, and separate means independently effective for directing magnetic flux through each of said magnetic paths.

12. A unitary magnetic structure for an electrical measuring instrument, said magnetic structure including a substantially cylindrical magnetic core, a magnetic body substantially surrounding said magnetic core, said magnetic body having a pair of pole pieces spaced from said magnetic core to form therewith a pair of substantially annular air gaps, said magnetic core having a passage extending completelytherethrough to divide said magnetic core into two portions angularly spaced about the axis of said magnetic core, said magnetic structure including separate magnetic means connecting each of said magnetic portions to said magnetic body, and a separate winding surrounding each of said pole pieces for directing, when independently energized, magnetic flux into the associated air gap.

13. In an electrical instrument, coil means designed for rotation about an axis intermediate two sides thereof, said sides being positioned substantially equidistant from any of a plurality of points spaced along the axis, a magnetic structure, means mounting said coil means for rotation relative to said magnetic structure about said axis, said magnetic structure comprising a pair of magnetic parts each defining paths for magnetic flux produced by current flowing in said coil means, each of said magnetic parts being configured to provide a substantial force acting etween the magnetic part and said coil means in response to current flowing in said coil means, and means reversely mounting said magnetic parts to direct said forces acting on the coil means in opposition to each other, whereby movement of said coil means is substantially unaffected by said forces.

14. In an electrical instrument, a magnetic structure, coil means designed for rotation about an axis intermediatetwo sides of the coil means, said sides being positioned substantially equidistant from any of a plurality of points spaced along the axis, and means mounting said coil means for rotation with respect to said magnetic structure about said axis, said magnetic structure comprising a pair of magnetic parts each having an air gap for each of said two sides of said coil means, each of said magnetic parts defining magnetic paths for magnetic flux produced by current flowing in said coil means which are asymmetric with respect to the path of travel of said coil means, and means reversely mounting said magnetic parts with their asymmetries oppositely associated with said coil means to provide resultant magnetic paths linking each of said coil sides which are substantially symmetric with respect to the paths of travel of the associated coil sides.

15. In an electrical instrument, a magnetic structure, coil means substantially symmetric about an axis, and means mounting said coil means for rotation with respect to said magnetic structure about said axis, said magnetic structure comprising a pair of unitary magnetic parts each having an air gap for each of two sides of said coil means, each of said magnetic parts defining magnetic paths for magnetic flux produced by current flowing in said coil means which are asymmetric with respect to the path of travel of said coil, and means mounting said magnetic parts with their asymmetries oppositely associated with said coil means to provide resultant magnetic paths linking each of said coil sides which are substantially symmetric with respect to the path of travel of the associated coil side, and separate means for directing magnetic flux through each of said resultant magnetic paths.

16. In an electromagnetic device, a magnetic structure having inner and outer pole piece structures defining arcuate air ga-ps substantially symmetricallydisposed relative to an axis, means associated with said magnetic structure for producing a magnetic field in said air gaps, coil means, and means mounting said coil means with a separate coil side positioned in each of said air gaps for movement therethrough about said axis; said inner pole piece structure comprising a pair of first magnetic cores extending through said coil means, said first magnetic cores being asymmetric with respect to the path of movement of said coil means to produce a first force acting on said coil means when electrical current flows through the coil means, and a pair of second magnetic cores extending through said coil means, said second magnetic cores being positioned asymmetrically with respect to the path of movement of said coil means and reversely relative to the first magnetic cores to produce a second force acting on said coil means in opposition to said first force when electrical current flows through the coil means, said magnetic cores being configured to define a passage extending through said inner pole piece structure to permit insertion and removal of said coil means therethrough relative to said magnetic structure.

17. In an electrical instrument, a magnetic structure, coil means, and means mounting said coil means for'rotation relative to said magnetic structure about an axis, said axis being located intermediate two sides of said coil means, said magnetic structure comprising a pair of magnetic cores each having a separate part positioned between said axis and each of said sides of said coil means, a first one of said magnetic cores having a passage permitting movement of said coil means therethrough in a direction substantially parallel to said axis when said coil means is adjacent a first extreme of its path of rotation, and a second one of said magnetic cores having a passage permitting movement of said coil means therethrough in a direction substantially parallel to said axis when said coil means is adjacent a second extreme of its path of rotation.

18. In an electrical instrument, a magnetic structure, coil means, and means mounting said coil means for rotation relative to said magnetic structure about an axis, saidaxis being located intermediate two sides of said coil means, said magnetic structure comprising a pair of magnetic cores each having a separate part positioned between said axis and each of said sides of said coil means, a first one of said magnetic cores having a passage permitting movement of said coil means therethrough in a direction substantially parallel to said axis when said coil means is adjacent a first extreme of its path of rotation, and a second one of said magnetic cores having a passage permitting movement of said coil means therethrough in a direction substantially parallel to said axis when said coil means is adjacent a second extreme of its path of rotation, and said magnetic cores being spaced in a direction parallel to said axis sufficiently to permit movement of a side of said coil means therebetween.

19. In an electrical instrument, coil means, a magnetic structure, means mounting said coil means for rotation with respect to said magnetic structure about an axis, said axis being located intermediate a pair of sides of said coil means, said magnetic structure comprising four magnetic cores extending through said coil means, said magnetic cores being arranged in pairs located on opposite sides of said axisand spaced to permit movement of said coil means therebetween, and magnetic means connected only to one end of each of said magnetic cores, said magnetic means being connected to opposite ends of the magnetic cores in each of said pairs, said magnetic cores being so positioned that a side of said coil means simultaneously moves away from said one end of a first one of the magnetic cores passing through the coil means and towards said one end of a second one of the magnetic cores passing through the coil means.

20. In an electrical instrument, coil means, a magnetic structure, means mounting said coil means for rotation with respect to said magnetic structure about an axis, said axis being located intermediate a pair of sides of said coil means, said magnetic structure comprising four magnetic cores extending through said coil means, said magnetic cores being arranged in pairs located on opposite sides of said axis and spaced to permit movement of said coil means therebetween, the magnetic cores in each of said pairs being spaced along said axis for a distance sufiicient to permit movement of a side of said coil means therebetween.

21. In an electrical instrument, coil means; a plurality of substantially similar magnetic laminations each having a magnetic body and a pair of spaced, cantilever magnetic cores projecting in opposite directions from said body through said coil means; and means mounting said coil means for rotation about an axis passing between the magnetic cores of each of said laminations, at least one of said magnetic laminations being reversed about a line transverse to said axis with respect to the remainder of said magnetic laminations.

22. In an electrical instrument; coil means; a plurality of substantially similar magnetic laminations each having a magnetic body and a pair of spaced, cantilever magnetic cores projecting in opposite directions from said body through said coil means; and means mounting said coil means for rotation about an axis passing between the magnetic cores of each of said laminations, the free ends of said magnetic cores being spaced from said magnetic body to provide access for said coil means to the passage between said cores asosgcro through which said coil means may be passed in a direction substantially parallel to said axis, at least one of said magnetic laminations being reversed about a line transverse to said-axis with respect to the remainder of said magnetic la1ninations, said reversely related laminationsbeing spaced in a direction parallel to saidaxis sufficiently to permit passage of a side of said coil means therebetween.

23. In an electrical instrument; coil means; a plurality of substantially similar magnetic laminations each having a magnetic body and a pair of spaced, cantilever magnetic cores projecting in opposite directions from said body through said coil means, and a separate magnetic pole piece spaced from each of said magnetic cores to define with the associated magnetic core a magnetic path including an air gap, and means mounting said coil means for rotation about an axis passing between the magnetic cores ofeach of said laminations, said coil means having a separate coil side disposed for movement through each of the air gaps in each of said laminations, and separate means for directing magnetic flux through the magnetic paths associated with each of said coil sides, at least one of said magnetic laminations being reversed about a line transverse to said axis with respect to the remainder of said magnetic laminations.

24. In an electrical instrument; coil means; a plurality of substantially similar magnetic laminations each having a magnetic body, a pair of spaced, cantilever magnetic cores projecting in opposite directions from said'body through said coil means and a separate magnetic pole piece spaced from each of said magnetic cores to define with the associated magnetic core a magnetic path including an air gap; and means mounting said coil means for rotation about an axis passing between the magnetic cores of each of said laminations, the free ends of said magnetic cores being spaced from said magnetic body to provide access for said coil means to the passage between said cores through which said coil means may be passed in a direction substantially parallel to said axis, said coil means having a separate coil side disposed for movement through each of the air gaps in each of said laminations, and separate means for directing magnetic flux through the magnetic paths associated with each of said coil sides, at least one of said magnetic laminations being reversed about a line transverse to said axis with respect to the remainder of said magnetic laminations, said reversely related lame inations being spaced in a direction parallel to said axis suificien'tly to permit passage of a side of said coil means therebetween.

25. In an electrical instrument; a pair of units; each of said units comprising a unitary magnetic structure and coil means; said magnetic structure having portions extending through said coil means to define with the remainder of said magnetic structure a separate air gap for each of two sides of said coil means; and each of the units including a source of magnetomotive force for the air gaps; and means mounting said coil means for rotation as a single assembly relative to said magnetic structures about an axis extending between said two sides of each or said coil means.

26. In an electrical instrument; a pair of units; each of said units comprising a unitary magnetic structure and coil means; said magnetic structure having portions extending through said coil means to define with the remainder of said mag- 32 netic structure a separate air gap for each of two sides of said coil means; and each of the units comprising a source of magnetomotive force for the air gaps; and means mounting said coil means for rotation as a single assembly relative to said magnetic structures about an axis extending between said two sides of each of said coil means, said magnetic structures having passages extending therethrough to permit withdrawal of said coil means as a single assembly through said passages from operative position relative to said magnetic structures to a position external thereto.

27. In an electrical instrument; a pair of aligned units; each of said units comprising a magnetic structure and coil means; said magnetic structure comprising a first magnetic portion defining a first magnetic path having an air gap within which a first side of said coil means is positioned for movement, and a second magnetic portion defining a second magnetic path having an air gap within which a second side of said coil means is positioned for movement, said portions being spaced to define a passage communicating with said air gaps through which said coil means may be removed from operative position relative to said magnetic structure; and each of the units comprising a source of magnetomotive force for the air gaps; and means mounting both of said coil means for rotation as a single assembly relative to said magnetic structures about an axis positioned between said sides of each of said coil means, one of said coil means being proportioned for movement through both of said passages, whereby said coils may be removed as a single assembly from operative position relative to said magnetic structures.

28. In an electrical instrument; a pair ofunits; each of said units comprising a unitary magnetic structure coil means; said coil means being adapted for rotation about an axis intermediate two sides of said coil means with respect to said magnetic structure; said magnetic structure comprising a pair of pole pieces positioned on opposite sides of said axis, each of said pole pieces having a pole face adjacent the path of travel of a separate side of said coil means, a pair of magnetic cores projecting through said coil means on opposite sides of said axis, and a magnetic body connecting said pole pieces to one end of each of said magnetic cores, said ends of said magnetic cores being disposed on opposite sides of said axis, whereby each of said pole pieces cooperates with a separate oneof said magnetic cores to complete a magnetic path having an air gap within which a separate side of said coil means is disposed for movement, said magnetic cores being spaced in a direction trans-- verse to said axis to define a passage through which said coil means may be removed from 0perative position relative to said magnetic structure; and each of the units comprising a source of magnetomotive force for the air gaps; means mounting said units with the axes of said coil means in alignment; and means mounting both of said coil means for rotation as a single assembly about said axes.

29. In an electrical instrument; a pair of units; each of said units comprising magnetic structime including a substantially cylindrical magnetic core, a magnetic body substantially 'sur-' rounding said magnetic core, said magnetic body having a pair of pole pieces spaced from each magnetic core and spaced angularly about the axis of said magnetic core to form therewith a 

